The present invention relates to ferromagnetic thin-film structures exhibiting relatively large magnetoresistive characteristics and, more particularly, to such structures used for the storage and retrieval of digital data.
Many kinds of electronic systems make use of magnetic devices including both digital systems, such as memories, and analog systems such as magnetic field sensors. Digital data memories are used extensively in digital systems of many kinds including computers and computer systems components, and digital signal processing systems. Such memories can be advantageously based on the storage of digital symbols as alternative states of magnetization in magnetic materials provided in each memory storage cell, the result being memories which use less electrical power and do not lose information upon removals of such electrical power.
Such memory cells, and magnetic field sensors also, can often be advantageously fabricated using ferromagnetic thin-film materials, and are often based on magnetoresistive sensing of magnetic states, or magnetic conditions, therein. Such devices may be provided on a surface of a monolithic integrated circuit to provide convenient electrical interconnections between the device and the operating circuitry therefor.
Ferromagnetic thin-film memory cells, for instance, can be made very small and packed very closely together to achieve a significant density of information storage, particularly when so provided on the surface of a monolithic integrated circuit. In this situation, the magnetic environment can become quite complex with fields in any one memory cell affecting the film portions in neighboring memory cells. Also, small ferromagnetic film portions in a memory cell can lead to substantial demagnetization fields which can cause instabilities in the magnetization state desired in such a cell.
These magnetic effects between neighbors in an array of closely packed ferromagnetic thin-film memory cells can be ameliorated to a considerable extent by providing a memory cell based on an intermediate separating material having two major surfaces on each of which an anisotropic ferromagnetic memory thin-film is provided. Such an arrangement provides significant “flux closure,” i.e. a more closely confined magnetic flux path, to thereby confine the magnetic field arising in the cell to affecting primarily just that cell. This result is considerably enhanced by choosing the separating material in the ferromagnetic thin-film memory cells to each be sufficiently thin.
Operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector in such a film with respect to the easy axis of that film. Such an axis comes about in the film because of an anisotropy therein typically resulting from depositing the film during fabrication in the presence of an external magnetic field oriented in the plane of the film along the direction desired for the easy axis in the resulting film, or by the shape of the film favoring the magnetization thereof to lie along a preferred direction, or both. During subsequent operation of devices having this resulting film, such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the film magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the film's easy axis. The state of the magnetization vector in such a film can be measured, or sensed, by the change in resistance encountered by current directed through this film portion. This arrangement has provided the basis for a ferromagnetic, magnetoresistive anisotropic thin-film to serve as a memory cell.
In the recent past, reducing the thicknesses of the ferromagnetic thin-films and the intermediate layers in extended “sandwich” structures, and adding possibly alternating ones of such films and layers, i.e. superlattices, have been shown to lead to a “giant magnetoresistive effect” being present in some circumstances. This effect yields a magnetoresistive response which can be in the range of up to an order of magnitude or more greater than that due to the well known anisotropic magnetoresistive response.
The giant magnetoresistive effect involves a change in the electrical resistance of the structure thought to come about from the passage of conduction electrons between the ferromagnetic layers in the “sandwich” structure, or superlattice structure, through the separating nonmagnetic layers with the resulting scattering occurring at the layer interfaces, and in the ferromagnetic layers, being dependent on the electron spins. The magnetization dependant component of the resistance in connection with this effect varies as the sine of the absolute value of half the angle between the magnetization vectors in the ferromagnetic thin-films provided on either side of an intermediate nonmagnetic layer. The electrical resistance in the giant magnetoresistance effect through the “sandwich” or superlattice structure is lower if the magnetizations in the separated ferromagnetic thin-films are parallel and oriented in the same direction than it is if these magnetizations are antiparallel, i.e. oriented in opposing or partially opposing directions.
A memory cell based on the “giant magnetoresistive effect” can be provided by having one of the ferromagnetic layers in the “sandwich” construction being prevented from switching the magnetization direction therein from pointing along the easy axis therein in one to the opposite direction in the presence of suitable externally applied magnetic fields while permitting the remaining ferromagnetic layer to be free to do so in the same externally applied fields. In one such arrangement, a “spin-valve” structure is formed by providing an antiferromagnetic layer on the ferromagnetic layer that is to be prevented from switching in the externally applied fields to “pin” its magnetization direction in a selected direction. In an alternative arrangement often termed a “pseudo-spin valve” structure, the ferromagnetic layer that is to be prevented from switching in the externally applied fields is made sufficiently thicker than the free ferromagnetic layer so that it does not switch in those external fields provided to switch the free layer.
Thus, a digital data memory cell based on the use of structures exhibiting the giant magnetoresistive effect is attractive as compared to structures based on use of an anisotropic magnetoresistive effect because of the larger signals obtainable in information retrieval operations with respect to such cells. Such larger magnitude signals are easier to detect without error in the presence of noise thereby leading to less critical requirements on the retrieval operation circuitry.
An alternative digital data bit storage and retrieval memory cell suited for fabrication with submicron dimensions can be fabricated that provides rapid retrievals of bit data stored therein and low power dissipation memory through use of a cell structure that has a spin dependent tunneling junction (SDTJ), or magnetoresistive tunnel junction (MTJ), device therein based on a pair of ferromagnetic thin-film layers having an electrical insulator layer therebetween of sufficient thinness to allow tunneling currents therethrough. This memory cell can be fabricated using ferromagnetic thin-film materials of similar or different kinds in each of the magnetic memory films present in such a “sandwich” structure on either side of an intermediate nonmagnetic layer where such ferromagnetic films maybe composite films, but this intermediate nonmagnetic layer conducts electrical current therethrough based primarily on the quantum electrodynamic effect “tunneling” current mentioned above.
This “tunneling” current has a magnitude dependence on the angle between the magnetization vectors in each of the ferromagnetic layers on either side of the intermediate layer due to the transmission barrier provided by this intermediate layer depending on the degree of matching of the spin polarizations of the electrons tunneling therethrough with the spin polarizations of the conduction electrons in the ferromagnetic layers, the latter being set by the layer magnetization directions to provide a “magnetic valve effect”. Such an effect results in an effective resistance or conductance characterizing this intermediate layer with respect to the “tunneling” current therethrough. In addition, an antiferromagnetic layer against one of the ferromagnetic layers is used in such a cell to provide different magnetization switching thresholds between that ferromagnetic layer and the other by fixing, or “pinning”, the magnetization direction for the adjacent ferromagnetic layer while leaving the other free to respond to externally applied, fields. Such devices maybe provided on a surface of a monolithic integrated circuit to thereby allow providing convenient electrical connections between each such memory cell device and the operating circuitry therefor.
A “sandwich” structure for such a memory cell, based on having an intermediate thin layer of a nonmagnetic, dielectric separating material with two major surfaces on each of which a anisotropic ferromagnetic thin-film is positioned, exhibits the “magnetic valve effect” if the materials for the ferromagnetic thin-films and the intermediate layers are properly selected and have sufficiently small thicknesses. The resulting “magnetic valve effect” can yield a response which can be several times in magnitude greater than that due to the “giant magnetoresistive effect” in a similar sized cell structure.
As stated above, operating magnetic fields imposed externally can be used to vary the angle of the magnetization vector with respect to the easy axis in the ferromagnetic films of these various kinds of memory cell devices, particularly the free layers. Such operational magnetic fields imposed externally can be used to vary the angle to such an extent as to cause switching of the layer magnetization vector between two stable states which occur for the magnetization being oriented in opposite directions along the easy axis of the layer, the state of the cell determining the value of the binary bit being stored therein. One of the difficulties in such memories is the need to provide memory cells therein that have extremely uniform switching thresholds and adequate resistance to unavoidable interjected magnetic field disturbances in the typical memory cell state selection scheme used. This externally applied operating fields scheme is based on selective externally imposed magnetic fields provided by selectively directing electrical currents over or through sequences of such cells thereby giving rise to such magnetic fields so that selection of a cell occurs through coincident presences of such fields at that cell.
In such a coincident current selection arrangement, only that cell in the vicinity of the crossing location, or intersection, of these two paths (one over a sequence of cells and the other through another sequence of cells) experience sufficient magnetic field intensities due to the summing of the fields due to these two currents to cause such a magnetic state change therein. Cells in the array that are located far away from both of these two current paths are not significantly affected by the magnetic fields generated by such currents in the paths because such fields diminish in intensity with distance from the source thereof. Cells, however, located in relatively close proximity to one, but not two, of these two paths do experience more significant magnetic fields thereabout, and those immediately in or adjacent to one such path experience sufficient field intensities to be considered as being “half-selected” by the presence of current in that path intended to participate in fully selecting a different cell along that path at the intersection with the other path on which a selection current is present. Half-selection means that a bit is affected by magnetic fields from the current through one path but not another. Such a coincident interjected magnetic fields memory cell state selection scheme is very desirable in that an individual switch, such as that provided by a transistor, is not needed for every memory cell, but the limitations this selection mode imposes on the uniformity of switching thresholds for each memory cell in a memory make the production of high yields difficult.
As such magnetic thin-film memory cells are made smaller to thereby increase the cell density over the surface of the substrate on which they are disposed, the resulting cells become more subject to magnetic state, or data, upsets due to thermal fluctuations occurring in the materials therein. The depth of the energy well in the magnetic material of such cells can be approximated as Hweff*Ms*Volume, where Hweff is half the effective restoration magnetic field attempting to maintain the current magnetic state following perturbations thereto and so effectively providing the energy well depth, Ms is the saturation magnetization of the magnetic material in the cell, and Volume is the volume of the magnetic material in the cell. In conventional cells, Hweff is provided by shape anisotropy or anisotropy due to the material properties of the cell magnetic material, or both. Typically, the value of Hweff in these cells is less than 100 Oe.
The important factor is the smallest energy well depth for a half-selected cell. The design objective is to ensure that the memory cells are magnetically stable during the data storing, or magnetic state switching, procedure that is repeatedly undertaken with respect to other cells. However, the trade-off between thermal stability and magnetic stability is a serious problem when the total magnetic volume of bits is less than about 104 to 105 nm3. Although the required barrier energy to be thermally stable can be reduced by use of error correction and periodic retrievals with corresponding corrections of the data stored in the whole memory, such a memory takes additional chip area and increased processing steps thereby increasing costs and reducing operating rapidity.
Meeting the thermal stability requirement of maintaining the same energy well depth for thermal stability in scaled own area memory cells necessitates the thickness of the resulting free layer being increased. Then the total anisotropy field correspondingly increases and so the required sense line current through the cell increases and the required word line current adjacent the cell increases to be capable of switching the magnetization direction of the free layer. This in turn causes the current density in the sense line to increase and temperature to rise in the line. These results show the very dramatic increase in current density as cells are reduced in width so that electromigration in the current conductors along with heating must be considered.
As indicated above, a ferromagnetic layer and an antiferromagnetic layer can be deposited in succession so they are in contact with one another with the result that relatively large interatomic forces occur aligning electron spins (parallel for ferromagnetism and antiparallel for antiferromagnetism). These coupling forces at the interface between these layers can be such that the magnetization of the ferromagnetic layer is restored to its initial direction prior to being subjected to external magnetic fields even after very large external magnetic fields are subsequently applied thereto. Such external magnetic fields can be 1000 Oe or more, and the magnetization of the ferromagnetic layer will still be restored to its initial direction. Thus, if such an antiferromagnetic layer is provided in contact with a ferromagnetic layer in a memory cell so that relatively large coupling occurs therebetween, the energy well depth for a small memory cell can be greatly increased. Such an arrangement can increase the potential density of memory cells by more than a factor of 10 through permitting the cell dimensions to go from about 0.2 μm minimum dimensions to approximately 0.05 μm dimensions.
A film structure which exhibits even better resistance to the effects of large externally applied magnetic fields is provided by a compound ferromagnetic thin-film layer with an antiferromagnetic layer. This compound ferromagnetic thin-film layer is provided to have a net layer magnetization that, when fixed in orientation in the finally formed structure, will resist rotation of its magnetization so that the magnetization of this compound ferromagnetic thin-film layer will appear fixed in its orientation in the device, i.e. “pinned” in a direction relative to the finally formed structure.
This compound ferromagnetic thin-film layer is formed by depositing a ferromagnetic layer in the presence of an easy axis direction determination magnetic field, then a nonmagnetic layer of ruthenium (no orienting magnetic field needed in this instance) to provide a very thin ruthenium antiferromagnetic coupling layer. Thereafter, another ferromagnetic layer is deposited again in the presence of an easy axis direction determination magnetic field aligned as was the field for the first ferromagnetic layer. The resulting compound ferromagnetic layer has materials with high spin polarization in its outer layers due to the use of high magnetic induction ferromagnetic material therein, but has little net magnetic moment because of the ruthenium layer provided therebetween which strongly antiferromagnetically couples these outer layers through primarily exchange coupling (some magnetostatic coupling also present)so that the magnetizations of each are pointed in opposite directions. Thus, this layer is relatively insensitive to externally applied fields and contributes little to the spatial fields thereabout. However, the magnetization direction in this composite layer by itself is not very strongly fixed in any direction because of the relatively weak anisotropy exhibited by the ferromagnetic layers.
Thus, a further antiferromagnetic material “pinning” layer exhibiting a substantial magnetic anisotropy must be deposited on the last ferromagnetic layer in the presence of a magnetization axis determination magnetic field aligned with the fields used in forming the two ferromagnetic layers to strongly set the magnetization direction of the compound layer. Such an antiferromagnetic layer has a strongly fixed magnetization direction which, through exchange coupling to the last ferromagnetic layer on which it is deposited, strongly fixes the direction of magnetization of that layer also, and so that of the first ferromagnetic layer through the ruthenium layer. The result is an anti ferromagnetic layer coupled strongly to the compound layer together forming a “synthetic anti ferromagnet”.
The magnetic fields necessary to reach the layer switching thresholds to cause switching of the relatively fixed magnetization orientation layers magnetization directions for memory cells of smaller and smaller lengths and widths to thereby change the data stored therein have, of course, magnitudes beyond those of the fields required to switch the magnetization directions in the free layers of those cells which also increase for smaller cells as shown above. Generating such magnetic fields begins to require currents through such cells and associated word lines of magnitudes that result in current densities sufficient to cause significant electromigration of the conductive materials and operating temperature rises of the cell region which will alter device behavior and structure. Such effects thereby lead to a limit of some minimum size for these cells.
One possibility for avoiding such limits has been found through allowing memory cell device operating temperature increases due to heating because of supplying word line currents adjacent to, and sense currents in, memory cells sufficient to approach or exceed the Curie temperature of the ferromagnetic layers in memory cells without a “pinning” layer or layers therein, or to approach or exceed the blocking temperature of the antiferromagnetic “pinning” layer in cells having such a layer. Such word line and sense line current based temperature increases permit storage of information in those cells to be achieved without reaching current magnitudes otherwise necessary to switch the magnetization directions of the ferromagnetic layers. The direction of magnetization of the relatively fixed magnetization orientation layer such as the thicker ferromagnetic layer in a three layer “sandwich” structure can be selected by having a moderate magnetic field present oriented in the selected direction when the layer cools sufficiently below its Curie temperature for cells without a “pinning” layer present, or by a field sufficient to set the direction of the ferromagnetic layer adjacent an antiferromagnetic “pinning” layer when that “pinning” layer cools sufficiently below its blocking temperature for cells using such a “pinning” layer or, alternatively, a “pinning” layer composite. The blocking temperature of an antiferromagnetic layer is the temperature at or above which that layer loses its ability to “pin” the magnetization direction of an adjacent ferromagnetic layer below its Curie temperature which blocking temperature is usually less than the Nèel temperature of that layer. Similarly, the Curie temperature may not need to be fully reached to allow relatively easy reorienting of the magnetization direction therein.
FIG. 1 is a graph of a ferromagnetic material layer magnetization normalized by the layer saturation magnetization versus temperature normalized by the layer Curie temperature indicating the nature of Curie point based data storage. Magnetization as a function of temperature changes rapidly near the layer Curie point. If 0 to 100° C. is the ambient temperature range of the silicon chip supporting this layer, then self-heating currents would raise the temperature of the cell to the Curie point, approximately 200° C. The currents would track the silicon substrate temperature, i.e. there would be more current required if the substrate were 0° C. than if the substrate were at 100° C.
A similar scheme would apply if pinning through providing an antiferromagnetic layer were used instead, and the critical temperature would then be the Nèel temperature of the antiferromagnet. In an antiferromagnet, the ordering of magnetic moments of adjacent atoms are in opposite directions so that the net magnetic moment is zero or near zero. When an antiferromagnetic layer and a ferromagnetic layer are sputtered as sequential layers, the antiferromagnetic layer as indicated above can effectively pin the magnetization of the ferromagnetic layer with an effective field of thousands of Oersteds. This pinning field is used in the case of Nèel point data storage to provide the stability of the cell. As the cell is cooled to below the Nèel temperature, a small magnetic field should be adequate to write the cell into the desired memory state.
Reducing the magnitudes of currents necessary for causing the magnetically harder ferromagnetic layer in memory cells with or without a “pinning” layer to approach or reach its Curie temperature, or the antiferromagnetic layer in memory cells with a “pinning” layer arrangement to approach or reach its blocking temperature, and insulating such memory cells from their neighboring cells to provide good cell selectivity in storing information, requires providing some thermal isolation of each cell from its neighbors and from the integrated circuit substrate or any other kind of substrate serving as a heat sink. Such thermal isolation can be provided by use of electrical conductive interconnections that are of a relatively low thermal conductivity, and by supporting the memory cell on an electrical insulator of relatively low thermal conductivity.
As stated above, operating magnetic fields imposed externally by providing electric currents through the sense line in which a cell is connected, and through the word line adjacent to the cell, can be used to vary the angle of the magnetization vector with respect to the easy axis in the ferromagnetic films of these various kinds of memory cell devices. The various magnetoresistive memory cell types thus use a coincident interjected magnetic fields memory cell state selection scheme for retrieving stored data. Such magnetic selection schemes for data retrievals introduce further data disturbance opportunities through again generating magnetic fields that can combine with unwanted magnetic fields that appear.
Electrical currents along the same conductors, but of increased magnitude, can also be used to heat the cell to provide for either Curie point or Nèel point data storage. In operation, each cell, when not to be selected for a state imposition therein, has no currents in the pair of sense line and word line conductors crossing in or adjacent to them or has current in only one of that pair. A selected cell, on the other hand, coincidently receives both of the corresponding currents along the two perpendicular sense and word lines crossing in or by it and is thereby heated into the desired condition to set a selected memory state by therein by magnetic fields generated by one of these currents kept though reduced following the heating. However, this arrangement dissipates heat along the entire sense line and along the entire word line carrying these currents to thereby result in substantial power dissipation and heating of otherwise unselected cells. Thus, there is a desire for a memory cell configuration and corresponding memory cell selection arrangement that allows providing magnetoresistive based memories that are less dissipative but still having the memory cells therein provided with a large areal density.